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Banish your Wi-Fi woes

Posted on 15 Mar 2010 at 16:05

Paul Ockenden explains how to get the best out of your Wi-Fi connection

One thing that most Wi-Fi users don’t appear to understand is that not all of these 13 legally available channels can be used at the same time. The problem is the conflict between channel spacing and width: the channels may be 5MHz apart, but each one is actually 20MHz wide (even 40MHz wide in some 802.11n configurations and speed-boosted versions of 802.11g), so there’s considerable overlap between them.

In fact, the specification says to allow a 25MHz gap to avoid interference, but for practical purposes 22MHz is usually sufficient.

Because of this overlap, there’s room for only three channels to be active at any one time, and in practice users and manufacturers tend to choose channels 1, 6 and 11, although outside of the USA the combinations 1, 6, 12 and 1, 6, 13 are also employed.

The other channels can sometimes be used, since some degree of overlap with distant wireless networks is tolerable, but for maximum range and throughput you’re limited to those three concurrent channels within your own premises, or among neighbouring houses or flats.

Tools of the Wi-Fi wranglers’ trade

Those of you who are running Linux, or even Apple’s iPhone, will be overloaded with tools for monitoring local Wi-Fi usage and congestion, but Windows users have far less choice.

That’s not to say that there aren’t any good tools available; I really like inSSIDer, which is open source and available to download from MetaGeek. It’s a simple tool that graphically displays all the networks visible in your area, along with their relative signal strength.

It also records signal strength over a five-minute period, which is particularly useful in an office environment as laptops pop on and off the network, and intelligent access points adjust their transmission power to match.

I’ve fired up a copy of inSSIDer in several different home and SME environments where the owners were complaining of poor wireless speeds and frequent drops, only to find their wireless router working on exactly the same channel as several of their neighbours, and so all of them were competing for a time-slice of the same frequency.

It’s so easy to spot these kinds of problems with a tool such as this, and with a quick change of channel the wireless network suddenly starts working properly again.

Nearby wireless networks aren’t the only problem that can stop your Wi-Fi from working properly, though. As I mentioned earlier, microwave ovens also work in the same frequency band, and because Wi-Fi operates at such tiny signal levels, even the most well-shielded microwave oven will leak enough to affect nearby Wi-Fi kit. Even more problematic are video senders, certain wireless headphones and baby monitors, which may also share the 2.4GHz band.

Cheap devices are often so badly constructed that, as well as working at a particular frequency, they’ll also spit out various side harmonics and spurious signals that splatter across several Wi-Fi channels.

Even tools such as inSSIDer can’t help with problems such as this, because they’re designed simply to scan for valid Wi-Fi signals across the legal channel slots, and the WLAN card in your laptop doesn’t see all this spurious radiation, even though it’s affected by it.

Wi-Spy with my little eye

Once again MetaGeek comes to the rescue, this time with a commercial product. Its range of Wi-Spy devices are USB dongles that work as radio spectrum analysers and show all radio activity across the Wi-Fi bands, not just within the designated legal channels.

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